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'Black mold or toxic black molds are a hazardous health variety of micro fungus that usually grow in your buildings' damp areas—for example, bathroom, basement, kitchen, etc.'
Q1: In the above sentence, is the use of 'a' after 'are' appropriate? If so then why?
Q2: In this part' black mold or toxic black molds' isn't using 'toxic black mold' more appropriate instead of the plural form as they are referring to the same type of thing?
The rule is that a linking verb agrees with the subject, not the noun complement. This construction is rare but grammatical because most of the time the complement is singular or plural like the subject.
In this case, because the "molds" is the closest noun to the verb, the verb agrees with it and not with the "mold" that it is joined to with an "or."
It is conceivable that there are black molds that are not toxic but otherwise damaging, but I agree that the structure is confusing. Rephrasing it would be wise. However, saying "X or adjective X" is not ungrammatical.
